The present invention relates generally to missile guidance systems, and more particularly, to a method for measuring boresight and parallax errors between multiple missile track links, and for compensating missile guidance commands for these errors.
Missile guidance may involve multiple lines of sight. In conventional guidance systems, such as tube-launched, optically-tracked, wire-guided (TOW) guidance systems, an operator typically has a choice of two sighting systems to track a target. A missile is simultaneously tracked by two tracking subsystems, co-located with a telescope used by the operator. When tracking the target, the most effective sighting system to use under a given set of battlefield conditions is selected by the operator. For existing TOW guidance systems employing dual track capability, the operator has a choice of a "day" sight or a "night" sight. The day sight operates in the visible spectral region, either a direct view optical system or television system. The night sight operates in the far infrared spectral region. The line of sight is defined by a tracking reticle in a display viewed by the operator, in both sighting systems. The operator tracks the target by positioning the tracking reticle on the target.
The missile is tracked by two or more tracking sensors in existing TOW systems. A first tracking sensor operates in the near infrared spectral region. A second tracking sensor operates in the far infrared spectral region. Each sensor tracks the missile to the extent that it is capable in a particular environment. The sensors produce error signals proportional to the angular deviation of the missile from the line of sight. Logic in the guidance system determines which tracking sensor's output signals to use in guiding the missile based on the relative quality of data from each sensor.
Boresight errors between these lines of sight are a major factor in accuracy when guiding the missile to the target, particularly at long range. Parallax between the lines of sight can also affect accuracy. Present alignment concepts control the boresight errors by a combination of manufacturing tolerances, factory alignments, alignments by field service personnel, and operator adjustments to control the overall track link alignments. The final alignments are highly dependent on the accuracy with which various individuals make these alignments, and are susceptible to accidental misalignment.
A major limitation of present concepts is the final alignment between the operator's various tracking sensors. This is typically a field operation using a target of opportunity. The operator switches back and forth between tracking sensors and manually adjusts knobs until the target's position coincides in the fields of view of the tracking sensors. This manual operation provides an additional error source and introduces the real possibility of the operator's accidental introduction of large errors into the track loop. The usual assumption in system performance analysis is that this additional error source is comparable in magnitude to other error sources.
The effectiveness of the system ultimately depends on how well the tracking sensor used to guide the missile is aligned to the reticle of the sight that the operator uses to track the target. The alignment of the near infrared sensor to the day sight has been tightly controlled by a combination of manufacturing tolerances, and factory and field alignments, both manual and automatic. There is similar control of the alignment of the far infrared sensor to the night sight. These tolerances and alignments are sufficient to control overall alignment when the operator uses the day sight and guidance is developed from the near infrared tracker or when the operator uses the night sight and the far infrared is used for missile guidance.
When there is a cross-tracking situation, the alignment between the day and night sight becomes an error source. Cross-tracking occurs when the operator uses the day sight and guidance developed from far infrared data, or uses the night sight with guidance developed from near infrared data. This alignment is a manual adjustment that the operator can make at any time at his discretion. In performance analysis, assumptions are made as to the accuracy of this alignment. There is no guarantee that the operator will have made the alignment accurately. There exists a real possibility that the sights will be accidentally misaligned by large amounts. Accordingly, there exists a need for reducing boresight and parallax errors and improving system alignments.
It is an objective of the present invention to provide an improved method of measuring misalignment between multiple missile track links, and compensating guidance of a missile to a selected target. Another objective of the invention is the reduction of boresight errors when guiding the missile toward the target. A further objective of the present invention is the compensation for parallax errors in the tracking system. A still further objective of the present invention is to compensate for errors introduced manually into the tracking system.